1. Technical Field
The present invention relates to electrolyte compositions and solid polymer electrolytes for electric double-layer capacitors that are highly suitable for use in such applications as backup power supplies for various types of electronic equipment. The invention also relates to polarizable electrode-forming compositions and polarizable electrodes, as well as to electric double-layer capacitors arrived at using these compositions and components.
2. Background Art
Electric double-layer capacitors are currently employed as backup power supplies for computers memories. These capacitors, which make use of the electric double layer that forms at the interface between an electrode and a liquid electrolyte, have a small size, a large capacitance and a long cycle life.
Recent rapid advances in portability and cordless features in consumer electronic devices such as mobile phones have led to a heightened demand for electric double-layer capacitors. Because electric double-layer capacitors which use nonaqueous electrolytes have a higher voltage and energy density than those made using aqueous electrolytes, they are viewed as especially promising and have been the object of accelerated research efforts.
Secondary cells have hitherto been used for such applications. However, electric double-layer capacitors have come into widespread use as lower power consumption by electronic equipment has reduced backup current requirements, and because of the longer cycle life and broader service temperature range of the capacitors themselves.
Such electric double-layer capacitors have a construction in which a positive and a negative electrode (polarizable electrodes) are provided on a left-right pair of current collectors.
The positive and negative electrodes (polarizable electrodes) have been built by adding a conductive material to a large surface area material such as activated carbon to improve the conductivity of the electrode, and using a binder to support the mixture on a current collector such as aluminum foil.
In this case, a separator lies between the positive and negative electrodes. The electrodes and the separator are generally impregnated with an electrolyte solution.
However, in such prior-art electric double-layer capacitors, adhesion and tackiness between the pair of electrodes and the electrolyte (separator) is low. This has an especially large impact in film-type electric double-layer capacitors.
That is, film-type electric double-layer capacitors, as noted above, have a positive electrode/electrolyte (separator)/negative electrode construction. Unlike cylindrical capacitors in which the positive electrode/electrolyte (separator)/negative electrode composite is coiled and placed in a case, the absence of a coiling pressure in film-type capacitors means that pressure is not applied between the positive electrode and the electrolyte and between the electrolyte and the negative electrode, allowing the electrolyte to separate readily from the positive and negative electrodes. Thus, the electrolyte (separator) placed between the positive electrode and the negative electrode, in addition to serving as an electrolyte, must also have the ability to strongly bond the positive and negative electrodes. In other words, it must have adhesion and tackiness.
Examples of binders used in electric double-layer capacitors to support a slurry of a large surface area material such as activated carbon on a metal current collector include polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl pyrrolidone and carboxymethylcellulose. Of these, polyvinylidene fluoride has excellent film-formability.
However, none of these binders has the ability to dissolve ion-conductive salts to a high concentration or possesses itself a high ionic conductivity. Moreover, these binders are unable to simultaneously satisfy the requirements for strong adhesion to the current collector, low dc resistance, and large electrostatic capacitance.
In addition, to lower the interfacial resistance between the electrodes and the electrolyte (separator), it is desirable for the same polymer used in the electrolyte to serve also as the binder resin in the electrodes.
Such an electrode binder must retain the ability to bond large surface area materials such as activated carbon; that is, it must have tackiness and adhesion.
The binder resins and solid polymer electrolytes for electric double-layer capacitors that have hitherto been reported in the literature are all lacking in sufficient tackiness and adhesion, as well as other important properties. Hence, a need has been felt for further improvement.
The present invention was conceived in light of the above circumstances. A first object of the invention is to provide electrolyte compositions and solid polymer electrolytes for electric double-layer capacitors, which compositions and electrolytes are endowed with high ionic conductivity, high tackiness and shape retention. Another object of the invention is to provide polarizable electrode-forming compositions and polarizable electrodes having a high adhesion, god dielectric properties, and shape retention. A further object of the invention is to provide high-performance electric double-layer capacitors which can be arrived at us ng these compositions and components.
The inventors have conducted extensive and repeated investigations in order to achieve these aims. As a result, they have discovered that an effective way to raise the ionic conductivity within an electrolyte composition for electric double-layer capacitors composed primarily of a polymeric compound having oxyalkylene chain-bearing polyvinyl alcohol units and an ion-conductive salt is to increase the proportion per unit weight of polymeric compound in which polyoxyalkylene segments capable of dissolving an ion-conductive salt are introduced onto the polymer.
That is, a typical example in which polyoxyalkylene branched chains are introduced onto a conventional natural polymeric substance such as cellulose might involve the introduction of a 10-mole unit length polyoxyethylene group per cellulose unit. In this case, the molecular weight of the cellulose recurring units (C6H10O5) is 162 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441. Hence, the fraction represented by the polyoxyethylene groups, which are the portions of the polymer that dissolve the ion-conductive salt, relative to the unit weight of the resulting cellulose derivative (polyoxyethylene fraction) is given by the ratio 441/(441+161)=0.733.
By contrast, if a polymeric compound such as polyvinyl alcohol (PVA) having a unit molecular weight lower than natural polymeric substances such as cellulose is used as the backbone, given that the molecular weight of the PVA recurring units (CH2CH(OH)) is 44 and the molecular weight of the 10-mole polyoxyethylene groups ((CH2CH2O)10xe2x80x94H) is 441, a higher polyoxyethylene fraction of 441/(441+44)=0.909 is achieved. The higher polyoxyethylene fraction enables a greater amount of ion-conductive salt to be dissolved, in addition to which the molecule has a larger number of polyoxyethylene segments where ion migration occurs, increasing ion mobility. The inventors have found that a high ionic conductivity can be attained in this way.
Also, when a film-type electric double-layer capacitor is assembled so as to include a solid polymer electrolyte (separator), for the solid polymer electrolyte to additionally serve as the binder component in the electric double-layer capacitor, it must have both a high ionic conductivity, and the ability to bind powdery battery active materials. That is, it must be tacky. Moreover, film-type electric double-layer capacitors made with solid polymer electrolytes generally have a polarizable electrode/solid electrolyte (separator)/polarizable electrode construction. Unlike cylindrical capacitors in which this polarizable electrode/solid electrolyte (separator)/polarizable electrode composite is coiled and placed in a case, the absence of a coiling pressure in film-type electric double-layer capacitors means that pressure is not applied between each of the polarizable electrodes and the solid electrolyte, allowing the solid electrolyte (separator) to separate readily from the pair of polarizable electrodes. For this reason as well, the separator disposed between the pair of polarizable electrodes, in addition to serving as an electrolyte, must also have the ability to strongly bond the polarizable electrodes. In other words, it must have tackiness and adhesion.
Pursuing their investigations even further based on these findings, the inventors have additionally found that electrolyte compositions for electric double-layer capacitors comprising (A) a polymeric compound containing polyvinyl alcohol units of general formula (1) below and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3, and (B) an ion-conductive salt, as well as electrolyte compositions for electric double-layer capacitors comprising (A) the above polymeric compound, (B) an ion-conductive salt and (C) a compound bearing crosslinkable functional groups have the ability to dissolve a large amount of ion-conductive salt because of their high oxyalkylene fraction. Moreover, the presence in the polymeric compound molecule of a larger number of oxyalkylene segments over which the ions can migrate increases ion mobility, enabling a high ionic conductivity to be achieved. These electrolyte compositions also have a high tackiness which allows them to function as an excellent binder component capable of firmly bonding large surface area materials and conductive materials. The inventors have also discovered that solid polymer electrolytes which have a semi-interpenetrating polymer network structure wherein molecular chains on the polymeric compound (A) are intertwined with the three-dimensional network structure of a polymer obtained by crosslinking the above compound (C) having crosslinkable functional groups and which contain also the above ion-conductive salt (B) have a dramatically improved shape retention. Because the matrix is amorphous rather than crystalline, such solid polymer electrolytes have a high ionic conductivity and a high tackiness, making them ideal for use in electric double-layer capacitors. The inventors have found as well that electric double-layer capacitors arrived at using these compositions and components have an outstanding performance.
Accordingly, a first aspect of the present invention provides an electrolyte composition for electric double-layer capacitors, characterized by primarily comprising (A) a polymeric compound containing polyvinyl alcohol units of general formula (1): 
and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; and (B) an ion-conductive salt.
A second aspect of the invention provides an electrolyte composition for electric double-layer capacitors, characterized by primarily comprising (A) a polymeric compound containing polyvinyl alcohol units of above general formula (1) and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; (B) an ion-conductive salt; and (C) a compound having crosslinkable functional groups.
A third aspect of the invention provides a solid polymer electrolyte for electric double-layer capacitors, characterized by having a semi-interpenetrating polymer network structure wherein molecular chains on the polymeric compound serving as above component A are intertwined with the three-dimensional network structure of a polymer obtained by crosslinking the compound serving as above component C, and containing the ion-conductive salt serving as above component B.
A fourth aspect of the invention provides a polarizable electrode-forming composition for electric double-layer capacitors, characterized by primarily comprising (A) a polymeric compound containing polyvinyl alcohol units of above general formula (1) and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; (D) a large surface area material; and (E) a conductive material.
A fifth aspect of the invention provides a polarizable electrode-forming composition for electric double-layer capacitors, characterized by primarily comprising (A) a polymeric compound containing polyvinyl alcohol units of above general formula (1) and having an average degree of polymerization of at least 20, in which compound some or all of the hydroxyl groups on the polyvinyl alcohol units are substituted with oxyalkylene-containing groups to an average molar substitution of at least 0.3; (C) a compound having crosslinkable functional groups; (D) a large surface area material; and (E) a conductive material.
A sixth aspect of the invention provides a polarizable electrode produced by coating the polarizable electrode-forming composition according to the above fourth or fifth aspect of the invention onto a current collector.
A seventh aspect of the invention provides an electric double-layer capacitor comprising a pair of polarizable electrodes with a separator disposed therebetween, characterized in that the above-described electrodes are used as the pair of polarizable electrodes and the separator is composed of a separator substrate impregnated with an ion-conductive salt-containing solution.
An eighth aspect of the invention provides an electric double-layer capacitor comprising a pair of polarizable electrodes with a separator disposed therebetween, characterized in that the above-described electrodes are used as the pair of polarizable electrodes and the separator is composed of a separator substrate coated or impregnated with the electrolyte composition for electric double-layer capacitors of the above first or second aspect of the invention.
A ninth aspect of the invention provides an electric double-layer capacitor comprising a pair of polarizable electrodes with a separator disposed therebetween, characterized in that the above-described electrodes are used as the pair of polarizable electrodes and the separator is a solid polymer electrolyte layer composed of the electrolyte composition for electric double-layer capacitors of the above first aspect of the invention.
A tenth aspect of the invention provides an electric double-layer capacitor comprising a pair of polarizable electrodes with a separator disposed therebetween, characterized in that the above-described electrodes are used as the pair of polarizable electrodes and the separator is a solid polymer electrolyte for electric double-layer capacitors according to the above third aspect of the invention.